Most traffic accidents by automobiles are caused by the oversight of the driver due to the presence of blind corner or lack of attention. It is important for the drivers to accurately recognize road signs such as “stop”, or road information related to safety such as road surface sign indicating crossing, stop line, speed limit, and the like for safe driving. As a technique of recognizing the surrounding signs and notifying the same to the driver, a method of establishing a communication between the road surface and the vehicle, a method of performing pattern recognition on the image acquired by the vehicle exterior photographic camera, and the like are used.
In an image recorded with outside environment of a vehicle for performing pattern recognition, various objects such as buildings, roadside trees, advertising displays, pedestrians, and other vehicles exist other than the sign to be recognized. Such objects greatly affect the pattern recognition of the sign. For instance, pattern recognition might be inhibited if one part of the crossing is shielded by the preceding vehicle passing through the intersection, or one part of the white line is shielded by the pedestrian walking on the road of an urban area.
The shadow of the object also influences the recognition result of the pattern recognition of the sign. For instance, the road surface sign may not be recognized if photographed as if the contrast of one part of the road surface sign is significantly lowered by the shadow of the own vehicle that is traveling and the shadow of the road sign on the road surface during the day time.
This means that the information is completely missing on the pattern portion shielded by the object, and the information of the shielded portion cannot be obtained unless previous knowledge of the pattern shape etc. is used. On the other hand, if the contrast of the pattern is changed by the shadow, information related to the pattern shape is not completely missing, and thus the original pattern shape information can be restored.
In pattern recognition, it is known that the recognition rate can be enhanced when recognition is performed using the entire recognition object as much as possible compared to the case in which recognition is performed using only one part of the recognition object due to reasons of difference in vision. Therefore, it is important to specify the shadow of the object in the image, and perform flag recognition with being emphasized in the area with shadow (shadow area) or the area without shadow (non-shadow area) in order to perform a flag recognition process with high reliability.
The location of a light source needs to be specified to specify the shadow of the object in the image. A technique of acquiring location information of the sun and using the same in the control of a vehicle or in a car navigation system (hereinafter referred to as car navigation) is disclosed in patent document 1 and patent document 2.
In the system disclosed in patent document 1, a shield for shielding a specific site of a vehicle from the solar light is controlled by calculating the location of the sun from the angle of inclination, the advancing direction, the position, and the date and time of travel of the own vehicle.
The system disclosed in patent document 2 is a system of calculating the location of the sun from the position, the direction, and the date of travel of the own vehicle, and applying a display color, a display shape, and a display shade in accordance with the actual weather condition on the car navigation screen with respect to the display object such as the sun, the building, and the sign.
Patent document 1: Japanese Laid-Open Patent Publication No. 2005-8058 (paragraphs 0033 to 0043)
Patent document 2: Japanese Laid-Open Patent Publication No. 2002-310689 (paragraphs 0039 to 0049)